When it comes to asteroids' wreaking disaster on Earth, the real question is not if, but when. Two hundred or so large craters and a geological record stretching over billions of years provide ample evidence that, time and again, explosive impacts by asteroids or comets have devastated large parts of the planet, wiped out species and threatened the very existence of terrestrial life. Astronomers are all too aware that more large hulks are out there, hurtling through space, some of them ultimately destined to collide with Earth.
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